Process for dehydrogenation of hydrocarbons



March 20, 1945. w. A. scHLZE ET AL 2,371,850 PROCESS FOR THEDEHY-DROGENATION OF HYDROCARBONS vFiled. March 25. 1942 INVEN'ToRs: vWALTER A. scHuLzE HILLYER .STEAM f Si] BUTANE- BUTENE MIXED FEEDcatalysts and conversion conditions.

Patented Mar.v 20, 194'5 UNITED STATESPATENT oli-rice PROCESS FORDEHYDROGENATION F AHYDROGARIiQNS Walter A. Schulze and John C. lillyer,Battles ville, Okla., assignors to Phillips Petroleum Company, acorporation of Delaware Application March 23, 1942, Serial No. 435,895A

7 Claims.

This invention relates to the catalytic dehydrogenation of hydrocarbonsto produce olens and diolens. It relates more specifically to animproved process for the' production of low-boiling aliphatic diolelnsfrom the corresponding monooleiins.

In the catalytic,dehydrogenation of olefins to Y produce diolens withoutalteration of the carbon chain, the high temperatures of operation andthe reactive nature of both feed and product have necessitated thedevelopmentof highly selective Thus, in view of the effect oftemperature on the olefindioleiin equilibrium, it has proved desirableto operate at temperatures well within the range of thermaldecomposition and to suppress same as far as possible by employing lowpressures, high ow rates, large proportions'of inert diluent,particularly water vapor, along with specially compounded and/ordeactivated catalysts. The

latter have been developed to the point that their activity in promotingeach possible reaction of the hydrocarbons with which they are incontact has been studied and modified to conform to maximum efliciencyin diolefin production.

However, in spite of these developments which have greatly improved theeconomics of olefin dehydrogenato'n, one of the principal diiiicultiesolefin yields during the latter portion of the conversion period may becontrolledand/or eliminated by employing shorter periods and morefrequent catalyst reactivation. l

However, such an expedient has not aided in eliminating the low diolenyields in the initial portion of the conversion periodsince this portionof the period must be gone through following each reactivation. As aresult average process yields are lowered and much valuable olefin feedstock is consumed in readjusting the reactivated catalyst to its optimumactivity for dioleiin production. The unfavorable eect is evencumulative, since destruction of the diolen in the initial low-yieldperiod usually causes heavier carbon deposits and more frequentreactivation.

It is the principal object of this invention to provide a process forcatalytic dehydrogenation of olefins wherein the efficiency in diolefinproencountered has been the non-uniform results obtained during the spanof a conversion period. In particular, there have been apparentvariations in the activity and selectivity of the catalyst afterreactivation wlich have resulted in an unfavorable discrepancy betweenolen conversion and diolefin yield. In fact, in the initial portion ofthe conversion period, with freshly reactivated catalyst, the peakolefin conversion often produces the lowest diolefln yield of the entireperiod indicating an abnormal rate of destruction of the dioleflnformed. This unfavorable effect markedly reduces the average diolenyield over thek entire conversion period since it coincides with theperiod of highest-catalyst activity and may not be `entirely.compensated for by subsequent operation at a lower conversion level andhigher efficiency.`

The conversion is", of course. accompanied by a gradual accumulation ofcarbonaceous deposits which result in reduced catalyst activity so thatolefin conversion decreases with the length of the conversion period.`Eventually, the declining activity makescontinued .use of the catalystunof the catalyst. In our process, we pretreat the economic, and theconversion period is terminated A and the catalyst is reactivated byburning o i the carbonaceous deposits. Thus, declining dlreactivation.

duction is nearly at a maximum when the hydrocarbon. feed is firstcontacted with the-catalyst.`

Another object of this invention is to provide a process for the`dehydrogenation of olens in which destruction of the diolen product isvlargely suppressed in the initial portion of the conversion periodimmediately following catalyst Still another object is to reduce therate of carbon deposition when olefin feed stocks are contacted withfreshly reactivated catalysts in processes of the type described,whereby longer conversion periods are possible with more uniformconversion and high diolen yields throughout. These and other objectsand advantages will be evident from the following disclosure.

We Ahave now found that we are able to dehydrogenateI monoolens `to thecorresponding diolefins in conversion periods of increased length whilesubstantially avoiding the period of very low diolen yield andefficiency at the beginningof the period by means of a certainpretreatment catalyst by contacting it with a relatively heatresistantparamnic hydrocarbon gas, generally one with fewer carbon atoms than theolefin it is subsequently desired-to dehydrogenate. This pretreatmentisl conducted for a regulated time period at a temperature substantiallyin the range used for olefin dehydrogenation.

We have found that after such a pretreatment' the catalyst exhibitsslightly modied properties including somewhat improved dehydrogenatingactivity at the very start of the conversion period. Deleterious sidereactions are markedly less prominent, while substantially maximumyields of diolelins are obtained initially or are rapidly Vsion toundesirable product.

attained. We have found that less of the valuable dioleilnic product isdestroyed in the catalyst space subsequent to its formation, and less ofthe monoolefln feed is likewise destroyed by-conver- The consequentcontrolled total monooleiin conversion together with the increased yieldof diolefins produce a marked increase in the efficiency of the Aprocessfor diolen production.

Many concurrent reactions and/or changes in the catalyst may occurduring our pretreating step. While the exact nature and extent of allthe changes occurring and the mechanism by which the improvement of thecatalytic properties isbrought about is not fully understood, it isreasonably certain that some of the following changes may occur. Amongthese may be mentioned especially desorption of unreactive gases such asnitrogen, water vapor and the like from the surface of the catalyst, andperhaps, deposition of a fine layer of carbon on certain portions of thecatalyst surface. Further reactions, such `as reduction of highermetallic oxides which may be present in thecatal'yst to lower oxides,conversion and/or removal of acidic components of the catalyst, and thelike may also occur.

Desorption of the unreactive gases adsorbed on the catalyst surface isvery necessary before activity can attain its full value, since thedehydrogenation reaction apparently occurs only with hydrocarbonmolecules which are themselves adsorbed on the catalyst surface. Oxygenand nitrogen are both known to be 'adsorbed very strongly on many 'ofour preferred dehydrogenation catalysts, and require considerable timefor complete desorption. Both these gases are present in fresh catalystfrom contact with the air, and

tions of the catalyst surface.

ordinarily in reactivated catalyst from the reactivation gas. 'I'heinert, tightly held nitrogen, oxygen, carbon oxides and .othercomponents of reactivation gases are replaced during the pretreatment bythe paraiiinic hydrocarmn employed. These latter, being more looselyadsorbed,

are subsequently much more readily replaced by the reactive olefin gasesof relatively analogous v adsorption characteristics when thedehydrogenation is begun. Moreover, since we have found that at thetemperature employed, the parain 'hydrocarbons may undergo a certainvery small w degree of conversion, some oleflns and hydrogen may also beformed and adsorbed by the catalyst 'understood, which occur during ourpretreating during the pretreating step. Hydrogen has been found to bevery readily displaced by the olen feed subsequently used, and while theamount present may be small, the effects are nevertheless beneficial inadjusting the catalyst activity.

Entirely aside from mechanical blanketing effects, oxygen adsorbed inthe catalyst may have a further deleterious etl'ect, by reacting withhydroous organic oxygen containing compounds. Theseproducts,,'generall'y of an acidic nature, may have prior to contactingwith the hydrocarbon results in poor yields until this vapor is removed;In both the above cases, the pretreatment of our process serves to freethe catalyst from traces of water resulting from the reactivationprocess. However, the eiects of the present vprocess greatly surpass thebeneiits of dehydration alone.

The conversion of the pretreating paraiin hydrocarbons, while veryslight, may alsoresult in the deposition of a thin layer of carbonv onpor- This alters the catalyst to a greater or less degree, andapparently affects particularly those centers responsible for theundesirable destructive reactions. We have found that such a carbondeposit is not primarily responsible for any increase in dehydrogenatingactivity subsequently displayed, for such a layer deposited on aninertporous carrier displays but slight activity in olendehydrogenation. In fact, the carbon deposit isordinarily so slight onthe catalyst following our pretreatment as to be hardly detectable onvisual inspection except at relatively scattered points on the surfaceof random catalyst granules.

The presence ,of oxides of various metallic elements in minor quantitiesin mineral catalyst is well known, as for example, iron oxides, titaniumoxides, and the like in bauxite. The higher oxides of these elementssuch, as for example, ferrie oxide, are known to be catalysts forcracking reactions, and when present in dehydrogenation catalysts resultin reduced efficiency. These oxides may be reduced to the lower formduring our pretreating process in which form they are relativelyinactive in catalyzing undesirable reactions. However, it is consideredof great sgnliicance that'` the predicated reduction of the oxides inour process is not sufllclently drastic to produce the correspondingmetals, e. g., iron, which mightbe even more undesirable than thehigher'oxi'des.,

We believe that the improved results obtained by our process are not duesolely to any one of the above-mentioned effects but to the peculiarcombination of all, and perhaps others less clearly process. It is anadvantage of our process that all` the components of the catalyst aremodified and all the desired reactions brought about by a single,relatively simple operating step. We' have also found that due to thelower initial-rate of destruction of hydrocarbon, and the consequentlower rate of accumulation of carbonaceous deposits, the activity of thecatalyst declines less rapidly. We are able thereby to realize evenhigher composite yields of diolefins and eiiiciencies 4carbonconstituents ofthe vapors to produce vari- 00 causing therebyconsiderable additional losses of 05 these valuble products until theoxygen and/or acidic matter formed during reactionation are completelyexpelled. By our improved process this dimculty is obviated.

It is well known that many catalysts of the type 70 employed functiononly when very dry and in the absence of water vapor. Even when a.waterv resistant catalyst isV employed in the presence of water vapor,it has been found that the presence of" absorbed water vapor on thecatalyst surface 75 in the conversion thereto; or conversely. if wedesire, to increase materially the length of the practicable operatingcycle. f

In one specific embodiment, our invention comprises producing butadieneby dehydrogenation of normal butenes over a bauxite type catalyst whichhas been pretreated at or near dehydrogenation temperature with aparailin hydrocarbon chosen from the group methane through propaneinclusive, or mixtures thereof. The process may be more readilyunderstood by reference tothe drawing, whichis a diagrammatic view ofone form of apparatus in which our process may be carried out.

In the iigure, essentially paramn hydrocarbon' f 2,371,850 for butenedehydrogenation. The hot vapors then pass through 1ine-8 to catalystcases 9 containing a dehydrogenation catalyst. 'I'he hot vapors thenexit through line I2 and valve I0, valve Il being closed.

When the pretreatment step is completed, fresh olen-containing feed isadmitted through line I and steam through line 6. The ilow of parafn iscut off by closing valve and the hydrocarbonsteam mixture is admittedthrough valve 3 to heater 1, where it is heated to conversiontemperature. The hot vapors then pass through line 8 to catalyst cases 9where they in turn contact the pretreated dehydrogenation catalyst. Thetreated vapors exit through valve I I and line I4. Valve IIJ is closed.The hot vapors passing through line III may be chilled by waterinjection through line I3 if desired and .pass to condenser I5 whereinwater vapor is condensed and condensate removed through line 22. 'I'hehy- ,drocarbon vapors then pass through line I6 to diolen separator Ilin which diolefins are extracted and removed'through line I8. This maybe effected by any one of several conventional methods such as chemicalseparation, solvent extraction or the like. 'I'he residual vapors passthrough line l@ and leave the system by line 20. Provision is made toreturn all or a part of the hydrocarbon vapors of the proper boilingrange to the system through line 2| for further conversion if desired.In such a case hydrogen and other light vapors could be removed from therecycle portion by means of fractionators, and/ or conventionalarrangements of apparatus. i

In the operation of ou'r pretreatment step, we ordinarily prefer toemploy parafn hydrocarbons of at least one less carbon atom than theolen to be dehydrogenated and particularly paraflins of one to three butnot more than four' carbon atoms. In this way the catalyst chambers maybe maintained at substantially conversion temperature during thepretreating step since. the lower paraiins are sufficiently more'refractory than the olen to be treated and the degree 'of conversion ofthe pretreating gas is always very slight. The temperature, length ofpretreatment and time of contact employed are chosen with respect to thecomposition of the pretreating gas. Thus, if methane is used for thepretreatment, we may find a period of yup to about two'hours at 1200 F.and at a space velocity of 60G-700 volumes of gas per volume of catalystper hour to be satisfactory, while if propane is used in the sameinstance, a period of thirty minutes at a temperature of about ll75 F.and a similar ow rate may give equivalent results. In general, with thelower hydrocarbons somewhat longer llt ciently low temperature andhighflow rate be used for onlyA a short period.

While our process is particularly adapted to the improvement of theyields obtained when using catalysts comprising bauxite, or modiedbauxites, it is also applicable toa considerable variety of othercatalysts. Among these may be men-- tioned particularly the naturalminerals' and the water-resistant group of dehydrogenation catalysts.Bauxite treated with alkali, or with al kaline earth hydroxides,particularly that treated with lbarium and strontium hydroxides may verysatisfactorily be used for butene dehydrogenation. Brucite, and certainpreparations of mag` nesium oxide also are useful in4 olen dehydro-`genation, as are bauxite catalysts impregnated with some magnesiumoxide, or magnesium and barium oxides together. By the termwater-resistant as applied to the catalysts which may be employed inthis invention, We describe catalysts which are not poisoned by thepresence of more than. a trace of water vapor in the hydrocarbonundergoing treatment at the specied temperature. In addition to theabove mentioned mate- 4.rials certain other catalysts fall into thiscategory.l The oxides of zirconium and titanium are capable offurnishing satisfactorycatalyst in synthetic preparations aswell as incertain` natural mineral ores.

In the operation of our process, the charge stock is usually lpreparedin such proportions thatthe partial pressure of oleflns is less than oneatmosphere and ordinarily in the range 0.1 to 0.5 atmosphere. The volumeof diluent, usually water vapor, added is from as low as 10 to as muchas 90 or more percent of the total being regulated to maintain thepartial pressure of olefin at the desired value. A certain proportion ofthe correpretreatment is used at relatively higher tem- I perature thanwith the longer chain hydrocarbons, the exact values being dependentupon the.

catalyst used', degree of modification desired, and the like.

Ethane. while entirely suitable, is generally not so readily availableexcept in mixtures for example with methane, the predomina-nt component,Natural gas may often be employed very Both methane and propane arevery. desirable hydrocarbons for the pretreatment.

sponding paraiin, in this case n-butane, can be tolerated in thedehydrogenation charge stocky especially when using water-vapor asdiluent together with a water-resistant catalyst. In such a process,disclosed in our copending application. Serial No. 412,637, ledSeptember 27, 1941', olefns may be selectively dehydrogenated in thepresence of considerable quantities of parains, the dehydrogenation ofwhich latter is inhibited by the-water vapor. Other gases than watervapor may be employed as diluents in our process. Among those which may`be successfully used are nitrogen, carbon dioxide, and lower parafns,such as methane, propane, and the like. In these cases, relatively smallquantities of the corre-.

4 ing aliphatic oleiins to diolens temperatures in the range o F. to1300 F. are ordinarily employed. Flow ratesI used are between 1 and 10liquid volumes of olefin charge per hour per volumef catalyst. In terms'of the total vapor mixture charged to the catalyst, space velocities of500 to 5000 are satisfactory under proper conditions. The particularcombination of ow rate and temperature for a specific operation willdepend on the catalyst employed, the composition of the charge, and onthe degree of conversion desired.

Since dehydrogenation is an equilibrium reaction, a portion of thebutene will always remain unconverted, the y exact proportion dependingupon the equilibrium concentrations at the temperature employed and howclosely other conditions of now rate, catalyst activity and the likeallow the equilibrium to be approached.' Obviously, further diolenscould be obtained from the unconverted oleflns by recycling all or anydesired proportion of them. Normally hydrogen and all or a part pf theother light gases present in the residual stream will be removed beforerecycling to avoid repressing the dehydrogenation. Numerous arrangementsof conventional equipment may be used for this purpose.

The preferred catalysts prepared and/or selected by the methodsdescribed may be reactivated over long periods of use by treatment withoxidizing gases to burn out carbonaceous residues responsible fordecreased activity. In the reactivation of the preferred catalysts,temperatures above about 1400 F. are usually avoided since permanentinjury. to the catalyst might result.

Normally the reactivation of the catalyst requires somewhat less timethan the conversion period. This is more or less independent of thelength of the conversion period since usually the quantity of cokedeposited is less in a shorter period and less time accordingly is usedto burn it oil'. The catalyst must then be raised to and.

maintained at operating temperature until the chamber is again, put backon stream. The pretreatment of the present invention may serve thispurpose as well, since it is carried out at or near operatingtemperature. Our improved process may thus be operated very convenientlyand economically in the conventional equipment provided fordehydrogenation. In this way little, if any, added time is required forthe additional pretreating step. Consequently, since the E- stream timefor a catalyst case is not substantially increased, no increase in thesize or number of catalytic converters required for a given plant isnecessitated, and the full improvement in production of diolens from aunit is realized. The following examples will further illustratespecicapplicatlons of our process:

Example I A dehydrogenatlon catalyst comprising granular' calcinedbauxite impregnated with three weight per cent of barium hydroxide waspre- 800 volumes (NTP) per volume of catalyst per'` hour for two hours.

At the end of the pretreatment, dehydrogenation oi' butene-2 was begunat once. The butene-2 admixed with three volumes of steam was 'chargedto the catalyst at a total ow rate of the mixed vapors of 1300 volumesper hour. The dehydrogenating conditions were 1205 F. and a catalystinlet/pressure of about 4 pounds gage.

The operation was continued until the per pass conversion of'butenedropped to about 30 per cent of the butene charged-a period o! 12 hours..Results are tabulated below, showing the per pass conversion and yieldof butadiene, based on the butene charged, and the conversion efiiciency(yield/conversion x 100) minutes.

ser P u ene u ne er cen Tune hom converted converted eiliciency per passto butadiene i; 4s lao a7 Q3 18. 0 42 20. 0 50 39 19. 5 50 36 17. 5 4930 14. 0 47 These results show that the elciency in conversion tobutadiene was initially high and remained in the range of to 50 per centduring most of theperiod. The maintained catalyst activity and diolefinyield indicate excellent control of side reactions during the rst houror so of conversion.

In contrast, the same catalyst was employed under identical conditionsafter reactivation, ex-

cept that the pretreatment was omitted. The

operation was discontinued after six hours, when the per pass buteneconversion had dropped to 31 per cent. Results are tabulated forcomparison with those listed above:

. Per cent Per cent Tune, hours butano butene con- Per cent convertedverted to efficiency per pass butadiene 54 6. 0 l1 45 15. 5 35 4l 18.044 31 13. 0 42 (Discon- Y tinued) In this test while the latter portionof the period showed good results, the low emciency in the irst nourshows excessive destruction 'of butene and/or butadiene with consequentreduced production and appreciable shortening of the conversion period.

Example II A portion of the catalyst describedv in Example I waspretreated with propane vapor at 1185 F. and a ilow rate oi 950 volumesper volume oi catalyst per hour. The pretreating period was Subsequentuse of the catalyst to dehydrogenate butene-Z gave results essentiallysimilar to those obtained` with pretreated catalyst of Example I.

Example III Bauxite catalyst modified by addition of cata- Volume percent CH4.'. 88 (32H6 10 CsHa 2 C4Hm Trace I'he pretreatment was carriedout at 1190a F. and

a ilow rate of 900 volumes per volumev of catalyst per hour for a periodof 'one and one-half hours. When used subsequently to dehydrogenate abutene-butane-steam mixture, eillciency in butadiene production wasessentially similar to that described for the pretreated catalyst ofExample I.

Example IV The catalyst of Example III after pretreatment with naturalgas in the manner describedl was used to dehydrogenate pentene-2.

Conversion troduction of the olefin tially improve the .within the rangeof about proportion of an emciency in the nrst two hours of the testavel aged 40 per cent. The same cat alyst was used in a subsequenttestafter reactivation but without pretreatment. The eiilciency in the firsttwo hours of this later test averaged only 25 per cent.

While the foregoing emplary operations -have given speciiicillustrations of the operation of our process, many modifications arepossible tions are intended except as expressed in the following claims.y

We claim: A

1. A process for the catalytic dehydrogenation of low-boiling aliphaticolens having at least four carbon atoms per molecule to produce thecorresponding dioleflns which comprises passing said olens admixed withsuillcient inert vdiluent to produce oleiin partial pressures in therangeI of about '0.1 to about hydrogenation catalyst comprising bauxitebearing catalytic proportions chosen from the group consisting of bariumand strontium hydroxides at temperatures of about 1100 to about 1300 F.and flow rates of from 500 to 5000 gas volumes per volume of'catalystper 'i' hour, whereby the oleflns are partially converted to thecorresponding diolens, periodically interrupting the ow of olefin feedstock, reactivating the catalyst by treating same with anoxygencontaining gas to burn off carbonaceous deposits accumulatedduring a treating said reactivated catalyst prior to reinfeed stock bypassing therethrough a paraillnic hydrocarbon -gasiconsistingessentially ofparafdnic hydrocarbon of one to three carbon atoms tiallyshorter than the conversion period. Y

2. A process as in claim 1 in which the parafilnic hydrocarbon gasconsists essentially of methane. 3, A process as in claim 1 inwbich theparafilnic hydrocarbon gas consists essentially of propane. 4. A processas in claim 1 inwhich'the paraiilnic hydrocarbon gas consists gassubstantially free of hydrocarbons boiling above propane and of ole .Y

5. A process for the catalytic dehydrogenationt allow-boiling aliphaticolefin hydrocarbon stocks having at least four carbon atoms per moleculeto produce the corresponding 'dioleiins which comprises passing saidolenns at a.i ,emperature 1 100 F.- to about 1300 F. into contact with acatalyst comprising a major aluminum oxide and a minor descriptivematter and ex' 0.5 atmosphere over a deconversion period, and pre-` l attemperatures and` now rates substantially within the dehydrogena-v tionrange for a period time suiilcient to substancatalyst activity-butsubstane' initial catalyst efnciency within the broad scope of `theprinciples disclosed. Therefore, no limitaof a metal hydroxide ductionof butadiene corresponding dioleflns which comprises said olens admixedAwith sufficient inert diluent essentially of .natural prior toreintroduction oi the oleiin reed stock by proportion of a metal`hydroxide selected from thev group consisting of barium and strontiumhydroxides;l periodically interruptingfthe .flow of said olefin;regenerating the catalyst by oxidation of carbonaceous materialdeposited thereon; and

pretreating the catalyst following said regenera tion 'and prior to usein the eonversio'n'by passing a pretreatins gas consisting essentiallyof paramn hydrocarbon of one to three carbon atoms per comprises passingsaid tions of a metal hydroxide chosen from the group suillcient tosubstantially improve the catalyst molecule and substantially free fromdiluent gases, from hydrocarbons boiling above propane and from oleiinsinto contact with said catalyst at a temperature within the range ofabout 1100 to about 1300 .F. for a period oi time suiiicient tosubstantially improve the catalyst activity and in the production ofsaid diolens from said oleilns.

6, A process for the catalytic dehydrogenation of normal butenes toproduce butadiene which butenes admixed with sumcient water vapor toproduce a butene partial pressure'in the range of about 0.1 to about 0.5

atmosphere over a dehydrogenation catalyst comprising aluminum oxidebearing catalytic proporconsisting of barium and strontium hydroxides attemperatures of about 1 100 to about 1300 F: and'ilow rates of from 500to 5000 gas'volumes per volume of catalyst per hour, whereby the butenesare partially converted to the butadienes, periodically interrupting theow of butene feed stock, reactivating the catalyst by treating same withan oxygen-containing gas to burn oficiar-` bonaceous depositsaccumulated during a conversion period, and pretreating said reactivatedcatalyst prior to reintroduction of the butene -feed stock by passingtherethrough a paraiilnic hydrocarbon gas consisting essentially .ofparailin hydiocarbon of one to three carbon atoms per moleculeandsubstantially free from hydrocarbons boiling abovepropane and fromoleiins lat temperatures and flow rates substantially within thedehydrogenation range for a period oi' time activity and initialcatalyst eiliciency' in the profrom normal butenes but substantiallyshorter than the conversion period.

7. A process for the catalytic dehydrogenation of low-boiling aliphaticolens having at least four carbon atoms per molecule to produce thePassing to produce olen partial pressures in the range of about 0.1 toabout 0.5 atmosphere overa bauxite dehydrogenation catalyst attemperatures of aboutA 1100 to about 1300 F. and flow rates of from 500to 5000 gas volumes per volume-of cata lyst per hour, whereby the olensare partially converted to the corresponding diolefins, callyinterrupting the now oi olelin feed stock, reactivating the catalyst byytreating same l.with an oxygen-containing gas to'burn off carbonaceousdeposits accumulated during a conversion period', and pretreating saidreactivated catalyst passing therethrough' a paramnic hydrocarbon gasconsisting essentially ofparafiinlc hydrocarbon of one to three carbonatoms at temperatures and flow 4rates substantially within thedehydrogenationrange for a peribd oi time suflicient to substantiallyimprove the catalyst activity but substantially shorter than'theconversion period.

WALTER A. SCHULZE.

JOHN C. HILLYER.

periodi-

